The operation of a cryogenic air separation plant requires that high boiling contaminants, such as water, carbon dioxide and hydrocarbons, be removed from the feed air before entering a column. Water and carbon dioxide will freeze at the cryogenic temperatures associated with the air separation. Hydrocarbons constitute a safety hazard if there is a potential for accumulation in a boiling oxygen pool. One method for removing these contaminants uses a combination of reversing heat exchangers or regenerators and an adsorbent bed for final clean up. Reversing heat exchangers and regenerators work by freezing out contaminants and then periodically switching flow passages such that passages which were occupied with high pressure air now pass a low pressure waste stream going in the opposite direction. This waste stream is colder but being at low pressure it can evaporate the contaminants and clean the exchanger passages in readiness for the next flow reversal. These periodic switches result in a momentary loss of feed air flow to the column system since the passages at low pressure must be repressurized with air after the switch before being able to pass feed air to the column system. The magnitude of this upset is a function of the pressure ratio between the feed air and waste and the volume of the heat exchanger passages. The problem will typically be most severe with regenerators due to their large volume. The upset can be reduced by equalizing pressures but still a reduction in the feed air flow is experienced.
Most recently, adsorbent beds have been used to remove high boiling impurities from a feed air stream to a cryogenic air separation plant. These beds have the advantage that they do not require a substantial waste stream and therefore the feed air that comes in can be obtained essentially as two or three clean products. Two types of such prepurifiers are employed: Thermal Swing Adsorption (TSA) and Pressure Swing Adsorption (PSA). As the names imply, TSA depends primarily on heat to drive the adsorbed contaminants off the adsorbent whereas PSA uses differences in absolute pressure within the bed to cause the contaminants to desorb. Both are operated in a batchwise manner where air is passed through a clean bed and the bed loads up with contaminants which are then removed in a desorption step. Typically, two or more beds are used. To minimize the impact on the column system, the bed that is to come on line is pressurized for a period of a minute or so by bleeding air off the feed to the other bed. When the bed is at pressure all of the air is diverted to it. During this repressurization step the column experiences a reduced feed air flow. This is again a periodic disturbance that will cause an upset to the column. The frequency and magnitude of the upset will depend on the particular prepurifier system. Typically the upset will be greater with a PSA because the frequency of switching is on the order of minutes whereas that of a TSA is on the order of hours.
The periodic or occasional feed air flow disturbance into the plant causes liquid within the column or columns of the air separation plant to drain down the column internals and fall into the sump of the column, or at least to flow to a lower level within the column compared to steady operation, thereby causing the column to operate inefficiently, i.e. with a high height per equivalent theoretical plate (HETP).
Accordingly it is an object of this invention to provide a cryogenic air separation system which can operate efficiently despite periodic or occasional feed air flow disturbances into the plant.